1. Field of the Invention
The present invention relates to a scanning unit for an optical position measuring device. The scanning unit is suitable not only for generating incremental signals but also in particular for generating reference pulse signals.
2. Discussion of Related Art
Known incremental position measuring devices offer not only the generation of position-dependent incremental signals but usually also the capability of generating so-called reference pulse signals at one or more defined points along the measurement path. With the aid of the reference pulse signals, an absolute reference in the position measurement can then be furnished in a known manner.
The scanning unit of an incremental position measuring device, described in the publication by R. Burgschat, entitled xe2x80x9cDie neue Dimension der Weg- und Winkelmesstechnikxe2x80x9d [xe2x80x9cThe Novel Dimension in Travel and Angle Measurement Technologyxe2x80x9d] in FandM 104 (1996) 10, pages 752-756, also offers the capability of detecting reference pulse signals at one or more points of the measurement path. The scanning unit here includes a first photodiode array, which is used to generate the incremental signals; this photodiode array is also described in German Patent Application DE 195 27 287 A1. Spaced apart in the measurement direction from it and offset vertically from the measurement direction is a second photodiode array. The second photodiode array serves to generate the reference pulse signal; that is, in this way a reference marking on a side of a scale, which marking is disposed laterally adjacent to the incremental grating line in a line of reference markings, can be detected photoelectrically.
If the reference pulse signal is generated in this way, incorrect measurements can occur under certain circumstances. For instance, local soiling of the scale in the region of the line of reference markings can for instance lead to the generation of a reference pulse signal. Furthermore, if there is miscalibration of the scanning unit and scale about an axis perpendicular to the plane of the scale, the reference pulse signal is no longer assuredly in a location- invariant position relative to the incremental signals.
A scanning unit in accordance with the species, which avoids the above problems, was proposed in German Patent Application 199 21 309.7 and U.S. patent application Ser. No. 09/565,887, filed May 5, 2000. However, in connection with this scanning unit it has been found not to be optimal that in case of an undesired change of the scanning distance, i.e. the distance between the scanning unit and the scale, an uneven change in the incidence of light results, in particular on the compensation detector elements.
The compensation detector elements are used for generating a compensation signal, or a so-called constant light level, which is required for further processing of the different scanning signals. Thus, an uneven light incidence on the compensation detector elements results in errors in the further processing of the signals.
An object of the present invention is therefore to disclose a scanning unit for an optical position measuring device in which the above-discussed problems in conjunction with the generation of reference pulse signals are avoided as much as possible. A compact structure of the corresponding scanning unit is also desirable.
This object is attained by a scanning unit for an optical position measuring device, which is suitable for scanning a scale in a measurement direction. The scanning unit includes a light source, an incremental signal scanning arrangement, disposed symmetrical around the light source and including a plurality of incremental signal detector elements, which are each disposed relative to one another so that phase-offset partial incremental signals are generated from the scanning of an incremental grating line of a scale. A first reference pulse detector element and a second reference pulse detector element, each of which is disposed vertically to a measurement direction and are each adjacent to the incremental signal scanning arrangement and generate an output reference pulse signal from the scanning of a first reference marking and a second reference marking located at one or more defined reference positions of the scale.
Another aspect of the present invention that attains the above-mentioned object regards an optical position measuring device that includes a scanning unit as described above.
The provisions according to the invention now assure that any possible rotation of the scanning unit relative to the scale about an axis oriented perpendicular to the plane of the scale will not lead to an incorrect determination of the reference position. On the contrary, in this case as well, the stable phase relationship of the generated reference pulse signal relative to the incremental signals is assured.
Furthermore, because of the generation of the reference pulse signal according to the present invention, the insensitivity to soiling is markedly increased. While in a system with reference markings disposed on only one side adjacent to the incremental grating line, local soiling in this region can lead to the erroneous generation of a reference pulse signal, when the scanning unit of the present invention is used, no output reference pulse signal caused by possible soiling of the scale can be generated. The reason for this is the fact that according to the present invention, two reference marking tracks adjacent to the incremental grating line are scanned; only if partial reference pulse signals are actually detected in both reference marking tracks does any output reference pulse signal whatever result.
Furthermore, it can be noted that now both for scanning the incremental grating line and for scanning the reference markings on the scale, only a single light source is required. By comparison, the scanning unit described in the aforementioned Burgschat publication requires two separate light sources.
Overall, because of the provisions of the present invention, an extremely compact scanning unit for an optical position measuring device is obtained that can also be used in tight spaces.
It is naturally also possible to use the scanning unit of the present invention in both linear and in rotary position measuring devices.
Another object and advantage of the present invention regards a scanning unit for an optical position measuring device, wherein it is assured that an even change of the light incidence on the various compensation detector elements results, even in case of a possible change of the scanning distance.
The above object and advantage is accomplished by one aspect of the present invention that includes a scanning unit with a plurality of compensation detector elements, which are used for generating a compensation signal, wherein the plurality of compensation detector elements are arranged in such a way, that each of the centers of gravity of their corresponding areas are located on a circle whose center coincides with an optical axis of the light source.
The above aspect of the present invention assures that an even change of the light incidence on all compensation detector elements, and therefore of the constant light level, results, even in case of a possible change of the scanning distance caused, for example, by guidance tolerances.
This is assured in accordance with the above aspect of the present invention in that the compensation detector elements are arranged in such a way, that the centers of gravity of their areas are all located on a circle whose center coincides with the optical axis of the system. Here, the position of the optical axis is defined by the centrally arranged light source of the scanning unit. Reference pulse detector elements are preferably also arranged in such a way that the centers of gravity of their areas lie on the mentioned circle. Moreover, it has been shown to be advantageous if the total area of all reference pulse detectors is selected to be identical to the total area of all compensation detector elements. Furthermore, the insensitivity of the reference pulse signal generation to possible tilting of the scanning unit, already mentioned in German Patent Application 199 21 309.7, continues to be assured. The same applies to the insensitivity of the reference signal generation to possible dirt accumulation on the scale.
Further advantages and details of the scanning unit of the present invention will become apparent from the ensuing description of an exemplary embodiment of the scanning unit of the present invention as well as a plurality of evaluation circuit arrangements, in conjunction with the accompanying drawings.